MELT morsels and butter in top of double boiler over hot (not boiling) water. Do not cover. Prevent water from coming in contact with morsels. Stir until smooth. Let chocolate sit over hot water.
COMBINE flour and baking powder in very small bowl. Whisk egg and sugar in medium bowl until blended. Slowly stir in the chocolate mixture and vanilla extract. Stir in flour mixture. Stir in 2 tablespoons morsels, pistachios and zest. Refrigerate for 15 minutes.
MEANWHILE, PREHEAT oven to 350° F. Scoop or spoon rounded tablespoons of batter onto ungreased baking sheet. Sprinkle each dollop lightly with sea salt.
BAKE for 9 to 10 minutes or until outside is slightly cracked. Do not overbake; the cookies will continue to bake while cooling. Cool on baking sheets for 3 minutes; top cookies with additional morsels if desired. Remove cookies to wire rack to cool. Cookies should be soft and gooey in center.
VARIATION: Cookies can be made without the nuts or zest.